1. Field of the Invention
This invention relates to the art of solar energy and more particularly, to amorphous silicon solar cells with improved light absorption efficiency and reflection-preventing or anti-reflective effect.
2. Description of the Prior Art
For the fabrications of amorphous silicon solar cells, it is the usual practice to use indium tin oxide (ITO) as to transparent electrode because of the excellent conductivity thereof. However, superposition of a plurality of amorphous silicon layers on the transparent electrode is generally carried out by a plasma CVD method or the like in which a gas such as monosilane (SiH.sub.4) is decomposed under a low vacuum. The severe decomposition conditions will cause indium tin oxide to deteriorate. To avoid this, the ITO electrode is currently coated on one surface thereof with SnO.sub.2 which is stable against plasma though lower in conductivity than ITO. The SnO.sub.2 -coated transparent electrode has been evaluated with respect to electrical conductivity, stability against plasma, and transparency, and it has been generally accepted that the transparent electrode should preferably be as smooth as possible on the surfaces thereof. In this connection, however, according to our experiments where the surface condition of the transparent electrode was changed, it was found that the absorption efficiency of light was better for a rough surface than a smooth surface.
A typical prior art solar cell comprising plural amorphous silicon layers has such a construction as shown in FIG. 1. A solar cell S comprises a glass substrate 1, on which are superposed a transparent conductive film 2, a plurality of amorphous silicon layers, 3, 4, 5, and a metal electrode 6 in this order. The transparent conductive film 2 formed directly on the glass substrate has not only the function of withdrawing the output voltage of the cell to outside, but also the function of a anti-reflective film in order to reduce the reflectivity of incident light.
While the transparent conductive film made of indium tin oxide has a refractive index of about 2, the amorphous silicon layers 3, 4, 5 each have a refractive index of about 4.0. Because the transparent conductive film 2 has a refractive index value which almost satisfies the conditions for non-reflection, proper choice of the film thickness will result in a small reflectivity at a certain wavelength as will be particularly seen from FIG. 2.
However, while the conductive film has a reflection-preventive effect at a certain wavelength, it exhibits a large reflectivity of incident light at other wavelengths, with the disadvantage that incident light covering a wide range of wavelengths cannot be effectively utilized. The reason why the transparent conductive film in the construction of known solar cells has an anti-reflective effect only at a certain wavelength is due to the fact that the transparent conductive film is formed of a single layer. If the conductive film is made of a multi-layered structure comprising layers having different refractive indices, a wider range of wavelengths will be provided with the anti-reflective effect. This technique has already been carried out in camera lenses but application of the conductive film of the multi-layered structure requires a material having a refractive index, n, intermediate between those of the amorphous silicon layer and the transparent conductive film, i.e. 2&lt;n&lt;4. In addition, such a material should be conductive and optically transparent. Thus, choice of such material is difficult in practice.